Ultrasonically improved convective drying of peppermint leaves: Influence on the process time and energetic indices

Journal Pre-proof Ultrasonically improved convective drying of peppermint leaves: Influence on the process time and energetic indices

Davoud Ghanbarian, Mehdi Torki-Harchegani, Morteza Sadeghi, Abdollah Ghasemi Pirbalouti PII:

S0960-1481(19)31515-0

DOI:

https://doi.org/10.1016/j.renene.2019.10.024

Reference:

RENE 12393

To appear in:

Renewable Energy

Received Date:

13 March 2019

Accepted Date:

06 October 2019

Please cite this article as: Davoud Ghanbarian, Mehdi Torki-Harchegani, Morteza Sadeghi, Abdollah Ghasemi Pirbalouti, Ultrasonically improved convective drying of peppermint leaves: Influence on the process time and energetic indices, Renewable Energy (2019), https://doi.org/10. 1016/j.renene.2019.10.024

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Journal Pre-proof Ultrasonically improved convective drying of peppermint leaves: Influence on the process time and energetic indices Davoud Ghanbariana,*, Mehdi Torki-Harchegania, Morteza Sadeghib, Abdollah Ghasemi Pirbaloutic

a

Department of Mechanical Engineering of Biosystems, Shahrekord University, Shahrekord, Iran

b

Department of Biosystems Engineering, College of Agriculture, Isfahan University of Technology, Isfahan 84156-83111, Iran

c

Research Center for Medicinal Plants, Shahr-E-Qods Branch, Islamic Azad University, P.O. Box 37541-374, Fath Highway, Tehran, Iran

*Corresponding

author: Tel: +98(913)3815396; Fax: +98(38)34424428

E-mail Address: [email protected] (Davoud Ghanbarian)

Journal Pre-proof Abstract The aim of the present work was to assess the influence of high-intensity airborne ultrasound application on duration and energetic performance of hot air drying of peppermint leaves. To this end, drying experiments were conducted at constant air flow rate of 1 m s-1 and different temperatures (40, 50, 60 and 70 °C) without and with application of ultrasound power (90, 180, 270 and 360 W). The results showed that the maximum (170.84 MJ kg-1) and the minimum (63.36 MJ kg-1 ) specific energy consumptions belonged to convective drying at temperature of 40 °C and ultrasonic-assisted drying at 60 °C and 270 W, respectively. Energy efficiency varied from 1.41 to 3.69%. Depending on the power level, application of the ultrasound power at the air temperatures less than 70 °C reduced drying time and improved energetic performance of the process. The obtained findings in this research declare that acoustic power could be effectively used in combination with convective dryers to reduce duration and energy required for drying of heat-sensitive products. Keywords: Ultrasound power, Drying, Specific energy consumption, Energy efficiency, Peppermint

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Nomenclature Et

Total energy input to dryer (MJ)

Eevap

Energy consumed to evaporate moisture from drying samples (kJ)

hfg

Latent heat of vaporization (kJ kg-1)

M

Instantaneous moisture content (w.b.) of drying samples

M0

Initial moisture content (w.b.) of drying samples

mw

mass of evaporated water from the product (kg)

SEC

Specific energy consumption (MJ kg-1)

Tabs

Absolute temperature of drying air (K)

W

Mass of samples (g) at each weighting during drying process

W0

Mass of fresh samples (g) at inception of drying process

Ww

Instantaneous mass (g) of water of drying product

Ww0

Initial mass (g) f water of fresh product

ηe

Energy efficiency (%)

2

Journal Pre-proof 1. Introduction Drying, as a vital unit operation process, is often used as a final production step before selling or packaging the products in different industries such as chemical, polymer, food, bioproducts, pharmaceutical, etc. In fact, dehydration is a simultaneous mass and energy transfer process in which water is removed by evaporation from a solid, semi-solid or liquid material [1,2]. Generally, fresh harvested agricultural products have high moisture content levels affecting their physical, chemical, and nutrient quality during the post-harvest life period. The main objective in drying of agricultural products is decrement of the moisture content to a certain value ensures safe storage by microbiological stabilization, and also allows further processing operations [3]. In most food production chains, drying is a critical stage since, in addition to likely undesirable physical and chemical changes affecting the dried product quality in terms of its nutritional properties and texture, the process consumes a great amount of energy [4,5]. Recently, growing demands for high quality products as well as increasing environmental concerns and the need for reduced energy costs have resulted in numerous detailed works considering dehydration of food materials using different drying systems. Among industrial drying methods, due to simplicity in construction and facility in operation, convective dryers are widely used to dehydrate agricultural products [6]. However, in practice, the dryers pose some critical limitations and disadvantages mainly including inefficient consumption of energy, relatively long process times, and undesirable quality of the dried product. Generally, high latent heat of water evaporation as well as low thermal conductivity and considerable wasted thermal energy by the air flow are the main reasons for these shortcomings [7‒9]. To surmount such obstacles, combination of hot air flow and different additional energy technologies has been suggested [10]. Microwave and infrared powers are the main hybrid

3

Journal Pre-proof technologies used in several industries to provide greater time, cost, and energy efficiencies than convective heating. The potential of microwave and infrared radiations in drying process is evident since they are thermal energies. However, their application to dehydrate the heatsensitive and valuable products such as medical and aromatic plants faces to some limitations because of over warming and quality deterioration of the products [11]. Application of ultrasound power (USP) constitutes an attractive hybrid drying technology and has been an interesting topic in the recent years. Ultrasound is mainly kinetic energy comes from sound waves in frequency range of 20 kHz‒1 MHz and, due to non-thermal character, its application in combination with thermal drying systems is promising [12]. Ultrasound has been employed in drying process in two different ways. In some works, it has been engaged as a pre-treatment before drying for improving the dehydration kinetics and also achieving final product with better functional quality indices. Nowacka et al. reported that ultrasound treatment (in the ultrasound bath at a frequency of 35 kHz) caused reduction of the drying time of apple slice by 31% in comparison to untreated tissue [13]. Romero et al. evaluated the use of ultrasound as a pretreatment for convective drying of Andean blackberry (Rubus glaucus Benth), and found that the power resulted in better quality indices of the dried product and reduced drying process time [14]. In the other mechanism, utilization of ultrasound power during drying have been investigated by many researchers. Beck et al. investigate the influence of airborne ultrasound conditions on dehydration behavior of the model food. They concluded that conventional hot air drying process could be significantly accelerated by using the airborne ultrasound [15]. Kowalski et al. reported that combined convective-microwave drying of raspberries (Rubus idaeusL.) was significantly improved in term of the process time as well as the product quality by application of ultrasound power [16]. In the different application forms of ultrasound power, a significant reduction in the process time is observed. In pre-treatment usage, by interrupting the cytomembrane

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Journal Pre-proof continuity, ultrasound power enhances the mass transfer from the cell to its extracellular environs [17]. While, the power application during drying process has been known helpful to enhance system efficiency through improving heat and mass transfer [17], and also to achieve better quality of products containing thermolabile constituents [10]. Acoustic streaming and generated cavitation are the main effects of ultrasound to augment heat and mass transfer rates. As well as, the heating and sponge effects are the subsequent effects of the ultrasonic waves decrease both the external and internal resistances to moisture transport, and consequently lead to higher drying rates [18]. As an applied energy source, acoustic energy has good potential for utilization in different industries. In last years, application of acoustic energy in combination with traditional technologies has gained great enthusiasts in the food industry due to its brilliant advantages such as no environmental impact as well as no quality deterioration of the products. However, scarce works have been reported in the open literature considering effect of ultrasonic power on energy use efficiency. Therefore, the main objective of the present study was to investigate the capability of ultrasonic-assisted convective drying to improve energetic performance in terms of the specific energy consumption and energy efficiency during drying process of peppermint leaves at different conditions. Quality attributes of the product as well as the design and structure of the ultrasonic combined dryer were not assessed. 2. Materials and methods 2.1. Experimental set-up and measuring instruments To study influence of the ultrasound power on performance attributes of hot air dryer, a pilotscale, ultrasonic-assisted convective dryer was designed and fabricated. Figure 1 presents a scheme of the combined hot air/ultrasonic dryer.

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Figure 1. A schematic of the ultrasonic-assisted convective dryer.

The drying set-up incorporates mainly an ultrasonic unit (including generator, transducer and vibrating cylinder), a heated air supplier, a process conditions (i.e., inlet air temperature and flow rate) controller, and a weighing system. The ultrasonic generator with a maximum power capacity of about 1500 W was composed of a power amplifier, a resonant frequency control system, and an impedance adapter. The drying chamber was an aluminium vibrating cylinder, with 125 mm height, 190 mm external diameter and 10 mm thickness, driven by a piezoelectric transducer (working at 20 kHz). To provide the system with a better electrical yield, using a matching unit, the impedance output of the ultrasonic generator was tuned to the transducer resonance frequency. A centrifugal fan blew air into an electrical heater and then, passed the heated air into drying chamber. Liner air flow rate was measured by using a thermal velocity probe with built-in temperature and humidity measurement, diameter of 12 mm, with telescope handle (maximum 75 mm) connected to a Testo AG 435 multi-function measuring instrument (0.01 m s-1 resolution, ±0.03 m s-1 accuracy, Testo SE & Co. KGaA., Lenzkirch, Germany). The air flow rate was adjusted by a 1–3 phase frequency inverter (TECO, S310-202-H1BCD, TECO Electric & Machinery Co. Ltd., Taipei, Taiwan). The temperature of the inlet drying air was measured using a Platinum RTD sensor (PT100, 0.1 °C resolution, accuracy of ± (0.15 °C + 0.2% of temperature)), and a PI controller (TPR2-N220V35AMR, HanYoung NUX Co., Ltd 6

Journal Pre-proof products, Korea) was implemented to control temperature of the air. Energy consumption of the heater was measured using a digital power meter (Ziegler Delta Power, accuracy of ±1% of nominal value, Ziegler instruments Co., Germany). During the experiments, the samples mass inside the drying chamber was measured using a precise electronic balance (Vibra, AJH-4200E, capacity of 4200 g, readability of 0.01 g, nonlibearity of ±0.01 g, Shinko Denshi Co., LTD., Japan) similar to the measuring system used by Gamboa-Santos et al. [11]. Furthermore, to protect the precision balance from air circulation and prevent measurement error, the balance was mounted on the outside top of the dryer and connected to the samples lot using a stainless steel screw rod. The system provided continuous monitoring the samples mass at preset time intervals without interruption. 2.2. Fresh samples The aerial parts of Peppermint (Mentha piperita L.) were harvested from a farm in Isfahan province (central Iran). To determine the initial moisture content of the peppermint leaves, 50 g of the fresh leaves was placed in an oven at 105 °C for 24 h [1]. This was repeated four times and the average initial moisture content of the samples was determined as 0.77 (w.b.). To preserve original quality and prevent moisture removal, the collected aerial parts were stored in a refrigerator at 4±2 °C until the drying experiments were started. 2.3. Drying experiments Drying conditions were selected for different combinations of drying air temperature (40, 50, 60 and 70 °C) and ultrasonic power (0, 90, 180, 270 and 360 W). The drying experiments were conducted in a laboratory room in which relative humidity of air remained constant at level of 13-15%. In each drying experiment, the dryer was run for at least 40 min to reach a steady state for the set points. About 170 g of the fresh leaves was placed in the drying chamber with a uniform height of 12 cm.

7

Journal Pre-proof The instantaneous mass of water of drying product can be evaluated as from the mass balance shown in Eq. (1): (1)

𝑊𝑤 = 𝑊𝑤0 ―(𝑊0 ―𝑊)

From Eq. (1), the instantaneous moisture content is derived by dividing by the instantaneous mass of drying product as represented in Eq. (2). 𝑀=

(

(𝑀0 ― 1) × 𝑊0 𝑊

)

(2)

+1

During the process, the samples lot was weighed accurately to 0.01 g and instantaneous moisture content of the leaves was computed using Eq. (2). All dehydration processes were continued until the samples lot reached to final moisture content of about 0.10 (w.b.) which is suitable for safe storage and also essential oil extraction. Each set of the experiments replicated three times and the average values were used. 2.4. Energy analysis The information obtained from the experiments was used to evaluate the energetic performance of the drying process regarding specific energy consumption and energy efficiency. Specific energy consumption of the process (MJ kg-1) was calculated using Eq. (3) [19]: 𝐸𝑡

(3)

𝑆𝐸𝐶 = 𝑚𝑤

Total energy consumption (Et) in the combined hot air-ultrasonic dryer was experimentally determined from the sum of energies consumed by electrical heater, centrifugal fan and ultrasonic generator. Applying the energy balance equation based on the First law of thermodynamics, energy efficiency for the drying process was calculated according to the following equations [19]: 𝐸𝑒𝑣𝑎𝑝

× 100

(4)

𝐸𝑒𝑣𝑎𝑝 = ℎ𝑓𝑔.𝑚𝑤

(5)

{

(6)

𝜂𝑒 =

𝐸𝑡

ℎ𝑓𝑔 = 2.503 × 103 ― 2.386(𝑇𝑎𝑏𝑠 ― 273.16) 273.16 ≤ 𝑇𝑎𝑏𝑠 ≤ 338.72 0.5 ℎ𝑓𝑔 = (7.33 × 106 ― 16𝑇2𝑎𝑏𝑠) 338.72 ≤ 𝑇𝑎𝑏𝑠 ≤ 533.16

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Journal Pre-proof 3. Results and discussion 3.1. Drying kinetics of the peppermint leaves Figures 2 and 3 illustrate typical dehydration curves for the peppermint leaves at some considering drying conditions. As shown (Figure 2), the process duration was decreased by rising the drying air temperature. For example, at constant USP of 180 W, the process duration at the temperatures of 40, 50, 60 and 70 °C was 275, 135, 78 and 58 min, respectively. Generally, the higher temperatures enhance heat transfer between thermal source and drying product resulting in facilitated moisture evaporation [20]. In addition, since interfacial concentration is a function of wet bulb temperature of the air, higher drying air temperatures increase the concentration of interfacial moisture of the product and cause higher driving forces for mass transfer and lower drying times [19]. The same results in terms of the influence of drying air temperature on drying time have been reported by Tulek for oyster mushroom drying in a cabinet-type convective dryer [21], Sadin et al. for infrared drying of tomato slices [22], Gamboa-Santos et al. for ultrasonic-convective drying of strawberry [11], Rodríguez et al. for thyme leaves in a power-ultrasound assisted convective dryer [23], Zare et al. for paddy in a combined hot air/infrared dryer [24], and Chen et al. for vacuum and ultrasonic-vacuum drying of carrot slices [10]. T= 40 °C

0.9

T= 50 °C

Moisture content (w.b.)

0.8

T= 60 °C

0.7

T= 70 °C

0.6 0.5 0.4 0.3 0.2 0.1 0 0

50

100

150 Drying time (min)

9

200

250

300

Journal Pre-proof Figure 2. Variation in moisture content of the peppermint leaves at different drying air temperatures USP=180 W. 0.9

USP= 0 W

Moisture content (w.b.)

0.8

USP= 90 W

0.7

USP= 180 W

0.6

USP= 270 W USP= 360 W

0.5 0.4 0.3 0.2 0.1 0 0

50

100

150

200

250

300

350

400

Drying time (min)

Figure 3. Variation in moisture content of the peppermint leaves at different ultrasonic powers for T=40 °C.

According to the obtained results (Figure 3), application of ultrasound power resulted in lower drying times where, in comparison with the conventionally hot air drying at constant air temperature of 40 °C, by applying USPs of 90, 180, 270 and 360 W, the process duration was shortened by 19, 30, 37 and 41%, respectively. Generally, the observation may be attributed to reduction of both the internal and external resistances to mass transfer due to mechanical effects of the ultrasonic waves. On the one hand, by the generation of oscillating velocities, microstreaming and pressure variation on the interfaces, the application of air-borne power ultrasound in solid/gas systems induces a mechanical stirring of the gas medium which reduces the boundary layer (external resistance to moisture removal) and consequently enhances the evaporation rate of moisture from the solid surface to the air [25]. On the other hand, acoustic energy also generates rapid series of alternating expansions and compressions (sponge effect) of solid material in which they are travelling. This continuous mechanical stress help to create microscopic channels which may facilitate the water movement from the product inner part to its surface [10]. Furthermore, high-intensity ultrasound power may also produce cavitation of water molecules inside the solid matrix which likely provide the energy required to change the water state and be operative to remove the strongly attached moisture [4]. From Figure 4, the 10

Journal Pre-proof increment in drying air temperature decreased the USP influence on the process time. For example, by applying the investigating USPs at temperature of 50 and 60 °C, drying time was reduced about 16, 27, 32 and 36%, and 11, 18, 23 and 26%, respectively. Also, at the air temperature of 70 °C, the application of USPs approximately caused no reduction in the process duration. It is worth noting that the effect of ultrasound power on drying time is dependent on some factor such as drying system and conditions as well as structure of drying products. Santacatalina et al. studied low-temperature drying of apple cubes and found that applying USP (50 W) decreased the drying time around 60% in the experiments carried out at 10, 5, 0 and -5 °C, and 77% in the experiments performed at -10 °C [4]. Rodríguez et al. investigated the influence of ultrasound power (45 and 75 W) on drying kinetics of apple cubes at constant air flow rate of 1 m s-1. They reported that applying the USP caused 37 and 54%, 35 and 46%, and 8.7 and 17.4% reduction in drying time at air temperatures of 30, 50 and 70 °C, respectively [26]. Gamboa-Santos et al. dried strawberry in a convective dryer by using air temperatures of 40, 50, 60 and 70 ºC at constant flow rate of 2 m s-1. They reported average drying time reductions by applying USP (30 and 60 W) ranged from 13 to 44% where the effect increased with increasing USP level and diminishes at high temperatures [11]. Also, under same drying treatments (air temperature of 40 °C, air flow rate of 1 m/s and ultrasonic powers of up to 90 W), García-Pérez et al. [27], Ortuño et al. [28], Ozuna et al. [29], Cárcel et al. [30] and GarcíaPérez et al. [31] reported 30, 53, 45, 40 and 72% reduction in drying time for lemon peel, orange peel, potatoes, carrots and eggplant, respectively.

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Journal Pre-proof 450

USP=0 W

400

USP= 90 W USP= 180 W

Drying time (min)

350

USP= 270 W 300

USP= 360 W

250 200 150 100 50 0 40°C

50°C 60°C Drying air temperature (°C)

70°C

Figure 4. Average time for ultrasonic-assisted convective drying process of the peppermint leaves moisture at different drying conditions.

3.2. Specific energy consumption Specific energy consumption (SEC) for the ultrasonic-assisted convective drying process of peppermint leaves was calculated using Eq. (2), and the obtained values are presented in Table 1. Table 1. Average specific energy consumption (SEC) for ultrasonic-assisted convective drying process of the peppermint leaves at different drying conditions. Drying air temperature (°C)

Ultrasonic power (W)

SEC (MJ kg-1)

40

0

170.84

90

152.81

180

143.69

270

141.88

360

143.94

0

101.57

90

92.07

180

86.27

270

85.52

360

86.02

50

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Journal Pre-proof 60

70

0

70.44

90

66.85

180

64.86

270

63.98

360

64.51

0

63.36

90

63.86

180

66.47

270

69.08

360

71.69

As shown, the specific energy consumed to dry the leaves ranged from 63.36 MJ kg-1 (for temperature of 70 °C and USP of 0 W) to 170.84 MJ kg-1 (for temperature of 40 °C and USP of 0 W) which is comparable with the values reported for various biological products in different drying systems and conditions (Table 2). Table 2. Some reported specific energy consumption (SEC) values for drying process of different biological products. Product

Drying system

Drying conditions

SEC (MJ kg-1)

Researchers

Berberis fruit

Hot air dryer of the static-

Air temperatures: 50, 60 and

75.38–3996.25

Aghbashlo et al.

tray type

70 °C

[32]

Air flow rates: 0.5, 1 and 2 m s-1 Pomegranate

Convective dryer

arils

Air temperatures: 45, 50, 55,

182.81–908.39

Motevali et al. [33]

5.02–20.74

Motevali et al. [9]

60, 65 and 70 °C Air flow rates: 0.5, 1 and 1.5 m s-1

Roman chamomile

Microwave-vacuum dryer

Microwave powers: 130, 260, 380 and 450 W Pressure: 25, 250, 500 and 750 mbar

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Journal Pre-proof Convective dryer

Air temperatures: 40, 50 and

69.76–251.87

60 °C Air flow rates: 0.5, 1 and 1.5 m s-1 Apple

Eletrohyrodynamic dryer

Air flow rates: 0, 1, 3 and 5 m

18–210

s-1

Martynenko et al. [34]

Voltage: 0, 5, 10 and 15 kV Wormwood

Convective dryer

leaves

Air temperatures: 50, 60, and

63.51–115.52

Beigi [35]

6.81–43.25

Tohidi et al. [19]

70 °C Air flow rate: 0.7 m s-1

Rough rice

Hot air dryer

Air temperatures: 40, 50, 60, 70 and 80 °C Air flow rates: 0.5, 0.8 and 1.1 m s-1 Air relative humidity: 40, 50, 60, and 70%

The differences among the values reported in the literature are due to the fact that consumed energy during drying process is mainly affected by drying system and conditions as well as the product properties. As shown in Table 1, for USPs of 0 and 90 W, specific energy consumption was decreased by increasing drying air temperature. Although rising the temperature leads to a decrement and increment in drying time and specific heat of air, respectively but the effect of shortened drying duration at higher temperatures is more significant than the effect of increased specific heat capacity. Therefore, higher drying air temperatures result in lower consumed energy [19]. The same observation have been reported in terms of the influence of drying temperature on energy consumption by Barzegar et al. for green peas [2], Zare et al. for paddy [24], Motevali et al. for Roman chamomile [9], Koyuncu et al. for azarole [36] and Alibas for nettle leaves [37]. It should be noted that, in spite of more efficient usage of energy, higher temperatures are not recommended to dry food and agricultural products due to likely 14

Journal Pre-proof undesirable changes in physical, mechanical, and chemical properties of the final product. Furthermore, from Table 1, increasing drying temperature from 60 °C to 70 °C with USPs of 180, 270 and 360 W resulted in slight increment in specific energy consumption of about 2.5, 7.96 and 11.13%, respectively. As presented in Table 1, depending on the power level applied, the application of ultrasound power combined with the convective dryer at the air temperatures less than 70 °C (40, 50 and 60 °C) decreased the specific energy consumption values compared to those for hot air drying alone. The observation could be directly ascribed to the shortened drying time resulted from application of the ultrasonic power. At these temperatures, the specific energy consumption decreased when USP increased up to 270 W. However, increasing USP from 270 to 360 W cause an increment in SEC values. In other words, from the SEC point of view, USP of 270 W had the best performance. Analysis of the obtained results revealed that, compared to hot air drying alone, USP of 270 W caused 17, 15.8 and 9.2% reduction in the process SEC at drying temperatures of 40, 50 and 60 °C, respectively. At temperature of 70 °C, using ultrasound power not only could not reduce the SEC values of the process but also increased it; where the application of USPs of 90, 180, 270 and 360 W resulted in 0.8, 4.9, 9 and 13.1% increment in SEC values of the process, respectively. In the open literature, scant works have been reported on the effect of the USP application on energy consumption of drying process in combination with other conventional dryers. Sabarez et al. developed an approach to use ultrasound in combination with convective dryer for transmission of ultrasonic energy as a combination of airborne and direct contacts to dry apple slices with 5 mm thickness at air temperature of 40 °C and ultrasound powers of 0, 75 and 90 W. Their results showed 46 and 57% reduction in drying time with possible 42 and 54% reduction in energy consumption, respectively [38]. Garcia-Perez et al. evaluated the influence of high-intensity ultrasound on energy consumption of orange peel drying by performing dehydration experiments at air temperature of 40 °C and

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Journal Pre-proof flow rate of 1 m s-1 without and with (45 and 90 W) ultrasound power application. They reported that, in comparison with the convective experiments, total energy consumed during the process was reduced by 12 and 20% for USP of 45 and 90 W, respectively [39]. Bantle et al. investigated energy consumptions for ultrasonic-assisted convective drying of clipfish for heat pump drying and heated ambient air drying at temperature of 20 °C, velocity of 2 m s-1, and relative humidity of 30% without and with (25 Watts per kilogram of wet product) application of ultrasonic power. They reported that heat pump drying without ultrasound was the most energy efficient dehydration process for clipfish (206 kWh ton-1) followed by heat ambient air drying without ultrasound (915 kWh ton-1). Despite of faster dehydration, the consumed energy for ultrasonic-assisted drying increased multiple times [12]. As mentioned before, depending on the process system and conditions as well as drying products properties, USP application in the process could lead to different results. For instance, as confirmed in the present work, the ability of USP to improve the time and energy consumption of the convective drying is more pronounced at low temperatures. The observation could be due to the enhanced obtainability of thermal energy at higher temperatures; hence the reduced contribution from ultrasonic power. 3.3. Energy efficiency The values of energy efficiency obtained for ultrasonic-assisted convective drying process of the peppermint leaves over the examined conditions are presented in Figure 5. Energy efficiency values ranged from 1.41 to 3.69%, where the minimum and the maximum values belonged to drying experiments at convectively drying only at temperature of 40 °C and ultrasonic-assisted drying at 60 °C and USP of 270 W, respectively.

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Energy efficiency (%)

3.5 3

USP= 0 W USP= 90 W USP= 180 W USP= 270 W

2.5

USP= 360 W

2 1.5 1 0.5 0 40°C

50°C 60°C Drying air temperature (°C)

70°C

Figure 5. Average energy efficiency for ultrasonic-assisted convective drying process of the peppermint leaves at different drying conditions.

The energy efficiency values determined in this work are comparable with those reported for convective drying of apple slice (2.87-9.11%) [40], chamomile (1.91-6.76%) [9] and grape stalk (6.1-7.2%) [6]. Long process time is the main reason for high energy consumption and low energy efficiency of convective dryers [41]. In convective drying, convection and thermal conduction phenomena are the governing mechanisms for heat transfer from the warm air to surface and into the interior of the product, respectively. Therefore, temperature inside the material cannot be increased rapidly and the process time is restricted by specific heat, thermal conductivity, density and viscosity. Furthermore, drying of agricultural and food materials takes place in falling rate period in general. Therefore, although the main portion of the water content is removed during the initial stages but, elimination of the remaining moisture needs more time [9]. From Figure 5, generally, higher air temperatures resulted in more values of energy efficiency. For instance, for convective drying only (USP= 0 W), the efficiency for temperatures of 40, 50, 60 and 70 °C was determined to be 1.41, 2.35, 3.35 and 3.68%, respectively. The observation well agrees with the findings reported by Motevali et al. for convective drying of chamomile at air temperatures of 40, 50 and 60 °C and air velocities of 0.5, 1 and 1.5 m s-1 [9] 17

Journal Pre-proof and Beigi for hot air drying of apple slices at temperatures in the range of 50-70 °C and flow rates in the range of 0.5-1.5 m s-1 [40]. For USP of 90 W, the same trends were also obtained. However, for the other considered USPs (180, 270 and 360 W), increasing drying temperature up to 60 °C improved energy efficiency, but at higher temperature (70 °C) the efficiency decreased. The observation is due to the more pronounced influence of ultrasound power on the process duration at lower temperatures, especially less than 70 °C, as before described. Similar to SEC, in comparison with hot air drying alone, applying ultrasonic power at temperatures of 40, 50 and 60 °C resulted in more efficient use of energy. At these temperatures, the USP of 270 W had the best performance, where the values of energy efficiency were specified to be 1.70, 2.79 and 3.69%, respectively. Furthermore, applying ultrasound power at temperature of 70 °C reduced energy efficiency of the process. At this temperature, the efficiency for USPs of 0, 90, 180, 270 and 360 W were determined to be 3.68, 3.65, 3.51, 3.37 and 3.25%, respectively. The simulator results reported by Garcia-Perez et al. for drying of grape stalk showed that, for drying experiments at 40 °C, the application of USP at 45W caused a reduction of 33.5% in the drying time compared to drying without USP, which provided an increment of 13% in energy efficiency. However, doubling the USP power from 45 to 90 W decreased the energy efficiency from 6.9% to 6.5%. Similar trend has been observed for drying experiments at 60 °C, where the energy efficiency was reported to be 6, 7.2 and 6.7% for USPs of 0, 45 and 90 W, respectively. Finally, from an energy point of view, they concluded that the experiments with USP application were more efficient at 45W than at 90 W [6]. 4. Conclusions The capability of high-intensity acoustic energy on duration and energetic performance of convective drying process of peppermint leaves was studied. According to findings obtained in the present study, ultrasound energy can be used effectively to enhance the conventional hot

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Journal Pre-proof air dryers in terms of reduction in the process time and energy requirements. The influence of ultrasonic power on the process time and energy consumption depend on drying air temperature and the applied power level. The lower the temperature, the more significant effect of the power. Combining the ultrasonic power by hot air at temperature of 70 °C, without any reduction in the process duration, increased the energy consumption. Acoustic power has good potential to be combined with industrial and commercial hot air dryers especially for heatsensitive products such as medicinal and aromatic plants. References [1] Torki-Harchegani M, Ghanbarian D, Ghasemi Pirbalouti A, Sadeghi M. Dehydration behavior, mathematical modelling, energy efficiency and essential oil yield of peppermint leaves undergoing microwave and hot air treatments. Renew Sust Energ Rev 2016;58:407–18. [2] Barzegar M, Zare D, Stroshine RL. An integrated energy and quality approach to optimization of green peas drying in a hot air infrared-assisted vibratory bed dryer. J Food Eng 2015;166:302–15. [3] Doymaz I. Thin-layer drying characteristics of sweet potato slices and mathematical modelling. Heat Mass Transfer 2011;47:277–85. [4] Santacatalina JV, Rodriguez O, Simal S, Carcel JA, Mulet A, Garcia-Perez JV. Ultrasonically enhanced low-temperature drying of apple: Influence on drying kinetics and antioxidant potential. J Food Eng 2014;138:35–44. [5] do Nascimento EMGC, Mulet A, Ascheri JLR, de Carvalho CWP, Cárcel JA. Effects of high-intensity ultrasound on drying kinetics and antioxidant properties of passion fruit peel. J Food Eng 2016;170:108–18. [6] Garcia-Perez JV, Carcel JA, Simal S, García-Alvarado MA, Mulet A. Ultrasonic intensification of grape stalk convective drying: kinetic and energy efficiency. Dry Technol 2013;31:942–50.

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Journal Pre-proof [7] Boudhrioua N, Giampaoli P, Bonazzi C. Changes in aromatic components of banana during ripening and air-drying. LWT – Food Sci Technol 2003;36:633–42. [8] Wang J, Sheng K. Far-infrared and microwave drying of peach. LWT – Food Sci Technol 2006;39:247–55. [9] Motevali A, Minaei S, Banakar A, Ghobadian B, Khoshtaghaza MH. Comparison of energy parameters in various dryers. Energy Convers Manage 2014;87:711–25. [10] Chen ZG, Guo XY, Wu T. A novel dehydration technique for carrot slices implementing ultrasound and vacuum drying methods. Ultrason Sonochem 2016;30:28–34. [11] Gamboa-Santos J, Montilla A, Cárcel JA, Villamiel M, Garcia-Perez JV. Air-borne ultrasound application in the convective drying of strawberry. Food Chem 2014;128:132–39. [12] Bantle M, Eikevik TM. A study of the energy efficiency of convective drying systems assisted by ultrasound in the production of clipfish. J Clean Prod 2014;65:217–23. [13] Nowacka M, Wiktor A, Śledź M, Jurek N, Witrowa-Rajchert D. Drying of ultrasound pretreated apple and its selected physical properties. J Food Eng 2012;113:427–33. [14] Romero J. CA, Yépez V. BD. Ultrasound as pretreatment to convective drying of Andean blackberry (Rubus glaucus Benth). Ultrason Sonochem 2015;22:205–10. [15] Beck SM, Sabarez H, Gaukel V, Knoerzer K. Enhancement of convective drying by application of airborne ultrasound – A response surface approach. Ultrason Sonochem 2014;21:2144–50. [16] Kowalski SJ, Pawłowski A, Szadzińska J, Łechtańska J, Stasiak M. High power airborne ultrasound assist in combined drying of raspberries. Innov Food Emerg Technol 2016,34:225– 33. [17] Chemat F, Zill-e-Huma, Khan MK. Applications of ultrasound in food technology: processing, preservation and extraction. Ultrason Sonochem 2011;18:813–35.

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Journal Pre-proof [18] Yao Y. Enhancement of mass transfer by ultrasound: Application to adsorbent regeneration and food drying/dehydration. Ultrason Sonochem 2016;31:512–31. [19] Tohidi M, Sadeghi M, Torki-Harchegani M. Energy and quality aspects for fixed deep bed drying of paddy. Renew Sust Energy Rev 2017;70:519–28. [20] Beigi M. Thin layer drying of wormwood (Artemisia absinthium L.) leaves: dehydration characteristics, rehydration capacity and energy consumption. Heat Mass Transfer 2017;53:2711–8. [21] Tulek Y. Drying kinetics of oyster mushroom (Pleurotus ostreatus) in a convective hot air dryer. J Agr Sci Technol 2011;13:655–64. [22] Sadin R, Chegini GR, Sadin H. The effect of temperature and slice thickness on drying kinetics tomato in the infrared dryer. Heat Mass Transfer 2014;50:501–7. [23] Rodríguez J, Mulet A, Bon J. Influence of high-intensity ultrasound on drying kinetics in fixed beds of high porosity. J Food Eng 2014;127:93–102. [24] Zare D, Naderi H, Ranjbaran M. Energy and quality attributes of combined hot-air/infrared drying of paddy. Dry Technol 2015;570–82. [25] García-Pérez JV, Cárcel JA, de la Fuente-Blanco S, Riera-Franco de Sarabia E. Ultrasonic drying of foodstuff in a fluidized bed: Parametric study. Ultrasonics 2006;44:539–43. [26] Rodríguez Ó, Santacatalina JV, Simal S, Garcia-Perez JV, Femenia A, Rosselló C. Influence of power ultrasound application on drying kinetics of apple and its antioxidant and microstructural properties. J Food Eng 2014;129:21–9. [27] García-Pérez JV, Cárcel JA, Riera E, Mulet A. Influence of the applied acoustic energy on the drying of carrots and lemon peel. Dry Technol 2009;27:281–7. [28] Ortuño C, Pérez-Munuera I, Puig A, Riera E, García-Pérez JV. Influence of power ultrasound application on mass transport and microstructure of orange peel during hot air drying. Phys Procedia 2010;3:153–9.

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Journal Pre-proof [40] Beigi M. Energy efficiency and moisture diffusivity of apple slices during convective drying. Food Sci Technol 2016;36:145–50. [41] Motevali A, Minaei S, Khoshtaghaza MH. Evaluation of energy consumption in different drying methods. Energy Convers Manage 2011;52:1192–9.

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Highlights 1- Ultrasound power ability to improve energy efficiency of drying process was studied. 2- Application of the power reduced the process duration of up to 41%. 3- At temperatures less than 70 °C, ultrasound power increased the energy efficiency. 4- The values of energy efficiency ranged from 1.41 to 3.69%. 5- The power has good ability to reduce the process duration and energy consumption.